Textured silicon substrates are used in the manufacture of photovoltaic and electronic products. Texture etching of polycrystalline silicon, referred herein as mc-silicon or silicon, is a key way of improving photovoltaic cell efficiency by trapping more light energy during photovoltaic cell operation than untextured silicon. A textured silicon surface minimizes light reflectance thus maximizing light absorption by the photovoltaic cell producing more energy. A textured silicon surface consists of minute recesses and projections on the planar level of the silicon surface, and is typically generated by an etching process.
Conventional texturing etch technology consists of treating mc-silicon with an etching solution. Etching solutions can be either caustic solution or acidic solutions. Caustic etching solutions typically are an aqueous solution containing an alkali earth metal, optionally with an alcohol, and are relatively slow when compared to acidic etching solutions. Acidic etching solutions typically contain a combination of acids, water, and optionally an additive. The concentrations of the acid components can be varied to alter the formation of a pattern of minute recesses and projections on the surface being etched, but such variations can also decrease the uniformity of the light trapping efficiencies of the various portions of the surface.
U.S. Pat. No. 6,340,640 of Nishimoto et al, discloses a method to produce a solar cell using an etch solution containing HF, HNO3, and an agent, wherein said agent contains at least one carboxylic acid with a molecular weight higher than acetic acid, or a mixture of phosphoric acid and a carboxylic acid with a molecular weight higher than acetic acid, to provide projections and recesses on a surface of the silicon wafer of a solar cell. The carboxylic acid is at least one of propionic acid, butyric acid, valoric acid, caproic acid, tartaric acid, succinic acid, adipic acid, propane-tricarboxylic acid, an isomer of propane-tricarboxylic acid. However, it is known that variations in the quality of the mc-silicon available make it difficult to realize efficiencies in mass production. The etching solution often needs to be customized to the quality of the mc-silicon available, requiring adjustments in the concentrations of acids used.
It is desirable to have a texturized silicon wafer with an increased number of surface projections and recesses in a continuous pattern of improved uniformity, and an increased surface density of such projections and recesses, while minimizing light reflectance. Also needed is a method to produce such a silicon substrate without decreasing etch rates acceptable to current manufacturing operations. The present invention meets these needs.